This invention relates to dielectric resonator apparatus functioning as a dielectric filter usable in a mobile communication device such as car telephones and portable telephones.
As an example of prior art dielectric resonator apparatus, FIG. 9 shows a three-stage bandpass filter having its main resonator part 1 formed with three 1/4-wavelength type dielectric coaxial resonators 2-4. Each of these dielectric coaxial resonators 2-4 has an outer conductor 5 in the form of a film electrode covering each external surface of a dielectric block in the shape of a rectangular parallelopiped except one of its end surfaces functioning as an open surface. The other end surface, on which the outer conductor 5 is not formed, serves as a shorted surface. Each of the dielectric coaxial resonators 2-4 has a throughhole 6 penetrating therethrough at its central portion, and an inner conductor is formed in the form of another film electrode on the inner surface of each of the throughholes 6. An approximately cylindrical metallic connector 7 is pressed into the interior of each of the throughholes 6 so as to be in an electrically conductive relationship with the inner conductor therein. A grounding electrode 14 in the form of a film is formed at a frontal part on the upper surface of a base board 11 made of a ceramic dielectric material. Another grounding electrode 14 is formed on the base board 11 towards the back. Although not visible in FIG. 9, still another grounding film electrode is formed on the lower surface of the base board 11 in electrically conductive relationship with the aforementioned ground electrodes 14 on the upper surface of the base board 11. Input/output terminals 12 and 13 are formed as film electrodes on both upper and lower surfaces of the base board 11 at its mutually opposite side parts. The parts of each of these input/output terminals 12 and 13 on the upper and lower surfaces are in electrically conductive relationship through conductive films over the side surfaces of the base board 11.
There is what will be referred to as a coupling board 21 made of a dielectric material, connecting the upper surfaces of the two input/output terminals 12 and 13. Capacitor-forming silver film electrodes 22-24 are formed on the upper surface of this coupling board 21, and another set of two capacitor-forming silver film electrodes 25 and 26 is formed at both ends on its lower surface, being connected by soldering to the input/output terminals 12 and 13 on the base board 11. The connectors 7 from the dielectric coaxial resonators 2-4 are connected, for example, by soldering to the capacitor-forming film electrodes 22-24 on the upper surface of the coupling board 21. It is to be understood that the film electrodes 25 and 26 are drawn in FIG. 9 to be thicker than realistic in order to show their existence and positions clearly.
As shown by the equivalent circuit diagram of FIG. 10, the dielectric resonator apparatus shown in FIG. 9 functions as a three-stage bandpass filter. A capacitor with capacitance C.sub.1 is formed on the side of the input/output terminal 12 between the film electrode 22 on the upper surface of the coupling board 21 and the film electrode 25 on the lower surface. Another capacitor with capacitance C.sub.1 is formed on the side of the input/output terminal 13 between the film electrode 24 on the upper surface of the coupling board 21 and the film electrode 26 on the lower surface. A left-hand side (as viewed in FIGS. 9 and 10) capacitor with capacitance C.sub.2 is formed between the film electrodes 22 and 23 and a right-hand side capacitor also with capacitance C.sub.2 is formed between the film electrodes 23 and 24 on the upper surface of the coupling board 21.
With reference next to FIGS. 11A and lib showing the structure of the aforementioned coupling board 21, it is to be noted that the capacitance C.sub.2 becomes large if the dimension a (or its width) is increased or the separations b between the film electrodes 22 and 23 and also between the film electrodes 23 and 24 are reduced. There are situations, however, wherein the dimensions of the coupling board 21 or the separations b cannot be changed freely although it may be desired to increase the value of the coupling capacitance C.sub.2. In other words, desired filter characteristics may not be obtainable in such situations.